1. Field of the Invention
The present invention relates to control of a so-called vibration type driving apparatus which uses an electro-mechanical energy conversion element to form traveling vibrations on an elastic body to relatively move the elastic body (a vibrating body) and a contact body.
2. Description of Related Art
A vibration type driving apparatus which uses an electro-mechanical energy conversion element to form traveling vibrations on an elastic body to drive a moving body (a contact body) is used as an actuator from which a large driving force at a low speed can be provided.
Particularly, Japanese Patent Application Laid-Open No. 2001-157473 has proposed a vibration type driving apparatus using traveling waves which excites a traveling vibration wave on an elastic body and continuously drives a moving body in press contact with the elastic body to allow more smooth driving.
In the vibration type driving apparatus described in Japanese Patent Application Laid-Open No. 2001-157473, a vibrating body is formed of an annular elastic body in which a group of protrusions in a comb shape is formed on one side of the elastic body in an axis direction. A friction material is bonded to the top surface of the group of protrusions. An annular piezoelectric element is bonded as an electromechanical energy conversion element on the other side of the elastic body in the axis direction, and a pattern electrode is formed on the piezoelectric element.
The pattern electrode is equally divided into electrode elements, the number of which is four times larger than the order of vibration modes excited in the annular portion of the vibrating body. The respective electrode elements are supplied with alternating voltages in a generally sine wave shape having time phases sequentially shifted 90 degrees. When an alternating voltage is supplied at a frequency near the natural frequency of an excited vibration mode, the piezoelectric element expands and contracts to provide bending moment for the elastic body to cause resonance of the elastic body. Vibrations (modes) excited by the alternating voltages having time phases shifted 90 degrees have the same shape and different phases. The vibrations are combined into a traveling vibration wave (a traveling wave).
FIG. 44 shows a driving circuit for driving a vibration type driving apparatus. The driving circuit is described in Japanese Patent Application Laid-Open No. 2002-176788, in which a switching circuit formed of MOSFETs 22 to 29 is controlled to turn on/off with a pulse generated by a pulse generating circuit, not shown, to produce an alternating voltage across transformers 30 and 31 with a center tap, thereby sequentially supplying alternating voltages with phases shifted 90 degrees to terminals 32 to 35 connected to secondary sides of the transformers corresponding to A(+), B(+), A(−), and B(−).
On the other hand, some of so-called standing wave driving type motors in which different vibrations (modes) are superimposed combine longitudinal vibrations with torsional vibrations as proposed in U.S. Pat. No. 5,777,424. In this example, the longitudinal vibrations and torsional vibrations are excited to have a phase difference of 90 degrees to use the longitudinal vibrations as vibrations for causing a vibrating body to separate from or come into contact with a moving body and the torsional vibrations as vibrations for carrying the moving body.
In such a vibration type driving apparatus driven by the superimposed different vibration modes, it is necessary to generally match the resonance frequencies in modes of different vibration directions in order to drive the modes of different vibration directions at the same frequency. However, the matching of the resonance frequencies is difficult because of anisotropy of the materials of the vibrating body and the like even when the vibrating body is formed in a uniform shape, and thus a frequency adjusting step is required.
In contrast, in the aforementioned so-called traveling wave vibration type driving apparatus driven by the superimposed vibrations (modes) of the same shape, the vibration modes have the same deformation distribution, so that the resonance frequency is unlikely to vary depending on the vibrating direction. Thus, almost no adjustment is necessary for matching the resonance frequencies in two modes.
The traveling wave vibration type driving apparatus, however, have the following problems since the vibrations (modes) of the same shape are superimposed.
FIGS. 45A and 45B schematically show a contact and driving state in a vibrating body (an elastic body) and a moving body.
FIGS. 45A and 45B show vibration displacement of a vibrating body 101 and response displacement of a moving body 106, and protrusions on the vibrating body and a friction material are omitted. Shown by solid line arrows in FIGS. 45A and 45B is driving vibration of the vibrating body 101 to drive the moving body 106 in a direction shown by outline arrows. FIG. 45A shows driving at a high speed with a large vibration amplitude, while FIG. 45B shows vibration in driving at a low speed with a smaller vibration amplitude than that in FIG. 45A. The smaller vibration amplitude as shown in FIG. 45B reduces the feed speed at each position to provide a lower speed (the speed is represented by the lengths of the outline arrows).
The moving body 106 is provided with bending rigidity and responsiveness such that a portion thereof is in contact with a portion of the vibrating body 101 where the feed speed is high, that is, where large displacement is produced. However, as the speed is reduced, the area of the vibrating body 101 in contact with the moving body 106 becomes larger, and finally, the vibrating body 101 is driven at a low speed with substantially the entire surface thereof in contact with the moving body 106 as shown in FIG. 45B.
When they are brought into contact in this manner, the efficiency is reduced since sliding friction acts on substantially the entire contact surface due to a partial difference in speed between the vibrating body and the moving body. In addition, wear powder produced on the contact surface is unlikely to be discharged to the outside and serves as grains to increase the wear amount of the moving body and the vibrating body.
Techniques for reducing the speed while the vibration amplitude is maintained to a certain degree include, for example as proposed in Japanese Patent Application Laid-Open No. 8 (1996)-80073, a method of switching to a standing wave at the time of stop, a method of changing to a standing wave by reducing a phase difference between an A phase and a B phase from 90 degrees, and a method of using a smaller vibration amplitude in one of the A phase and the B phase, although as a means, mainly for enhancing vibration responsiveness.
Such methods, however, have adverse effects on the contact surface between the vibrating body and the moving body.
For example, in the annular vibration type driving apparatus, a plurality of vibration modes which cause bending deformation of the vibrating body are superimposed with their positional phases shifted 90 degrees.
FIG. 46 is a developed view schematically showing vibrations of the vibrating body, and specifically, shows the vibrations when A(+), B(+), A(−), and B(−) of a piezoelectric element 102 are supplied with driving voltages having time phases shifted 90 degrees. Ellipses “a” to “g” shown in portions of the vibrating body 101 represent elliptical motions produced at positions of the vibrating body 101. Arrows shown in each ellipse show vibration components of the A and B phases constituting the elliptical motions (solid line arrows show the A phase and dotted line arrows show the B phase).
The vibration components of the A and B phases constituting the elliptical motions have varying directions depending on the positions. If the vibration amplitude of the A phase is reduced to produce a standing wave component, the longitudinal amplitude is reduced at some positions and the transverse amplitude is reduced at other positions to produce an uneven friction state. The unevenness leads to variations in wear speed of the friction surface to reduce flatness of the friction surface, causing degraded performance.
In addition, the maximum traveling wave vibration, that is, a large driving force, is always present at the same position. Thus, variations in surface pressure of the moving body and the vibrating body occur, or vibrations in rotation occur in synchronization with rotation of the moving body due to an uneven plane of the portion of the moving body in contact with the vibrating body, so that rotation accuracy may be reduced.